In recent years, the thermal transfer recording has come to draw attention in numerous fields specializing in facsimile devices, computer terminal devices, and recorders because it possesses numerous features including freedom from impact and noise, necessity for no maintenance, low cost, feasibility of reduction in size and weight, and adaptability to color recording. Among other methods, the method which effects electrothermal transfer by the use of a current-passing head suits a full-color recording containing intermediate gradation and deserves the keenest attention as a most promising approach to production of hard copies.
FIG. 1 is a diagram of operating principle illustrating passage of electric current through an electrothermal transfer recording sheet 1 by the use of a recording electrode 5 and a returning electrode 6. In this arrangement, thermal transfer recording on a given blank sheet for recording (not shown) is accomplished by pressing the recording head into contact with a resistive layer 2 of the recording sheet thereby starting flow of current therethrough and causing the resistive layer to generate and assumulate heat until an elevated temperature, and causing a support layer 3 to conduct heat to the ink layer 4 thereby enabling the ink layer 4 to be heated, melted, and made to flow.
The most important quality which the resistive layer in the electrothermal transfer recording sheet is expected to possess is such as to fulfil the requirements (1) that the magnitude of resistance should be lowered to about 10.sup.2 to 5.times.10.sup.5 ohms, (2) that the resistive layer should be given an ability to withstand heat above at least 300.degree. C. for a brief period, and (3) that the tight adhesion of the resistive layer to the support layer should be enough against the shear friction due to the forced contact made by the curring-passing head.
The resistive layers so far proposed invariably fall short of fulfilling these requirements.
Various devices which have been heretofore contemplated to fulfil these requirements will be described below.
The first problem concerns a reduction in the magnitude of resistance. In this case, for the resistive layer to generate heat by the flow of electric current therethrough, the magnitude of resistance offered by the resistive layer is required to an intermediate between the magnitudes of resistance offered by an insulating material and a good conductor. The magnitude of resistance is fixed by the balance of various factors such as the amount of power supplied, the thermal conductivity of the recording sheet, and the energy spent in melting the ink layer. As means to impart the resistive layer an ability to generate heat, a method which forms a resistive layer by dispersing powder of aluminum, copper, iron, tin, zinc, nickel, molybdenum, or silver as electroconductive particles in a resin binder, a method which effects the production of a resistive layer by dispersion of precipitated copper in a resin binder, a method which produces a resistive layer by dispersing zinc oxide or titanium dioxide in a resin binder, a method which obtains a resistive layer by applying an electroconductive polymer on a substrate layer, and a method which prepares a resistive layer by dispersing graphite or acetylene black in a resin binder have been proposed.
The inventors have studied all these methods. They have consequently found that relatively inexpensive electroconductive particles which exhibit high affinity for the resin binder enough to be uniformly dispersed in the form of finely divided particles within the binder and also exhibit high affinity for the solvent used in solving the binder resin and, therefore, are satisfactorily dispersed in the solvent are carbon type particles such as graphite and acetylene black. The carbon type particles include carbon black besides the aforementioned graphite. By the method of manufacture, the carbon black is divided under the furnace type, the channel type, and the thermal type. It also comes in numerous grades having varying particle properties. In all the carbon type particles available at all, graphite and acetylene black are excellent in electroconductivity. They are extensively utilized, as blended with polymers, in panel heaters, antistatic members, panel switches, and packaging materials.
The magnitude of voltage applied on the current-passing head can be decreased and, consequently, the capacities of the power source and the driving system for the head can be reduced and the electrothermal transfer device can be improved in reliability and economy in proportion as the magnitude of resistance offered by the resistive layer of the electrothermal transfer device is decreased. The magnitude of surface resistance offered by the resistive layer of 2 to 5 .mu.m in thickness is desired to fall in the range of 10.sup.2 to 5.times.10.sup.5 ohms, preferably 10.sup.3 to 10.sup.4 ohms. It has been demonstrated, however, that when the resistive layer is formed by using a conventional electroconductivity imparting filler such as graphite or acetylene black, it is difficult to lower the surface resistance below 5.times.10.sup.5 ohms. The surface resistance can be lowered to the order of 3.times.10.sup.5 to 5.times.10.sup.5 ohms by increasing the filling ratio of graphite or acetylene black. Then, the application of the resistance layer on the support layer will become difficult. Besides, the kinetic strength which the resistive layer exhibits under the forced contact of the current-passing head and the tight adhesion of the resistive layer to the support layer are too low for the resistance layer to withstand actual use.
The second problem concerns resistance to heat. It has been shown that, by the jourlean heat, the temperatures of the resistive layer and the support layer reach levels in the range of 150.degree. to 350.degree. C. and remain there, though for a brief duration of 20 .mu..sec to 20 m.sec.
As the material for the support layer, polyimide film or condenser paper is recommendable because of resistance to heat. Neither polyimide film nor condenser paper easily produces a film of small thickness in the range of 2 to 10 .mu.m. A film of small thickness of not more than 10 .mu.m required for transfer of a delicate intermediate gradation cannot be provided by such a material from the standpoint of thermal conductivity. Moreover, the polyimide film is too expensive to be used as a disposable material. In view of the balance of various factors such as thickness, heat resistance, and kinetic strength of the film, and price, there is no alternative but to select a biaxially oriented film of polyethylene terephthalate (PET) as the material for the support layer.
When the PET film is used as the support layer, however, there ensues the major problem that the PET film is melted along the path of the needle of the recording head and is consequently caused to sustain bores if the resistive layer offers poor resistance to heat. As the result, the image produced is seriously impaired in quality and, in an extreme case, is totally spoiled by sticking of the head needle.
Furthermore, as means of attaining tight adhesion of the resistive layer and the support film, there have heretofore been devised a method in which the PET film is activated by corona discharge and then coated with a resistive layer, and a method in which the PET film is undercoated with a thin layer of a modified polyester (0.3 to 1 .mu.m) and then coated with a resistive layer. These methods prove unfit because the former method fails to offer ample tight adhesion and the latter entails an addition to cost.